build: fix build errors/warnings in rare configurations

- vtls: fix unused variable and symbols.
- ftp: fix unused variable.
- http: fix unused variables.
- smtp: fix unsued variable.
- wolfssl: fix unused variable with !proxy.
- libssh: fix unused argument.
- curl_trc: sync guards between declaration and definition.
- curl_trc: add missing guard for `Curl_trc_ssls` when !verbose.
- curl_trc: fix errors with !http + http3.
- curl_trc: fix missing function with !http + nghttp2.
- cf-h2-proxy: disable when !http + nghttp2, to avoid calling undeclared
  functions.
- sha256: fix missing declaration in rare configs.
- md4: fix symbol conflict when building GnuTLS together with AWS-LC or
  wolfSSL. By prioritizing the latter two. AWS-LC has no option
  to disable the clashing symbol. wolfSSL does, but the most seamless is
  to skip including GnuTLS's standalone `md4.h` to avoid the clash.
- build: fix errors with !http + nghttp2.
- build: catch !ssl + ssls-export combination in source. Convert
  build-level errors to warnings.
- build: fix errors with !http + http3.
- build: fix building curl tool and unit1302 in rare combinations.
  By always compiling base64 curlx functions.

- cmake: add `_CURL_SKIP_BUILD_CERTS` internal option.
  To disable automatically building certs with the testdeps target.
  To improve performance when testing builds.
  (used locally to find the failing builds fixed in this PR.)

Closes #17962

CMakeLists.txt

@@ -1068,7 +1068,7 @@ if(USE_SSLS_EXPORT)
   if(_ssl_enabled)
     message(STATUS "SSL export enabled.")
   else()
-    message(FATAL_ERROR "SSL session export requires SSL enabled")
+    message(WARNING "SSL session export requires SSL enabled")
   endif()
 endif()